1. Field of the Invention
The present invention relates to a liquid crystal display device including plural sub-pixel electrodes in one pixel region and improved in alignment of liquid crystal molecules, and particularly to a liquid crystal display device in which at least one sub-pixel electrode is capacitance coupled to a control electrode to which a display voltage is applied, and is connected through a second TFT to a sub-pixel electrode directly driven by the control electrode in order to prevent image sticking.
2. Description of the Related Art
As compared with a CRT (Cathode Ray Tube), a liquid crystal display device has merits that it is thin, is lightweight, is driven by a low voltage, and has small electric power consumption. Thus, the liquid crystal display device is used for various electric equipments such as a television, a notebook PC (Personal Computer), a desktop PC, PDA (Personal Digital Assistant) and a cellular phone. Especially, an active matrix liquid crystal display device in which a TFT (Thin Film Transistor) as a switching element is provided for each pixel (sub-pixel) exhibits an excellent display characteristic comparable to the CRT because of its high drive performance, and has been widely used for the field in which the CRT is conventionally used, such as the desktop PC or the television.
In general, the liquid crystal display device includes two substrates, and a liquid crystal sealed between these substrates. A pixel electrode, a TFT and the like are formed for each pixel on one of the substrates, and a color filter opposite to the pixel electrode and a common electrode common to respective pixels are formed on the other substrate. There are three kinds of red (G), green (G) and blue (B) color filters, and a color filter of one of the colors is disposed for each pixel. Three pixels of red (R), green (G) and blue (B) disposed to be adjacent to each other form one picture dot. Hereinafter, the substrate on which the pixel electrode and the TFT are formed will be called a TFT substrate, and the substrate disposed to be opposite to the TFT substrate will be called an opposite substrate. A structure in which a liquid crystal is sealed between the TFT substrate and the opposite substrate will be called a liquid crystal panel.
Conventionally, a TN (Twisted Nematic) liquid crystal display device has been widely used in which a horizontal alignment type liquid crystal (liquid crystal with a positive dielectric anisotropy) is sealed between two substrates, and liquid crystal molecules are twist-aligned. However, the TN liquid crystal display device has defects that a viewing angle characteristic is poor, and the contrast and color tone are significantly changed when the screen is seen from an oblique direction. Thus, an MVA (Multi-domain Vertical Alignment) liquid crystal display device excellent in the viewing angle characteristic is developed and is put to practical use.
However, in the conventional MVA liquid crystal display device, there occurs a phenomenon in which a screen looks whitish when viewed from an oblique direction. FIG. 2 is a view showing a T-V (Transmittance-Voltage) characteristic when a screen is seen from the front, and a T-V characteristic when viewed from an upward direction of 60°, while the horizontal axis indicates the applied voltage (V) and the vertical axis indicates the transmittance. As shown in FIG. 2, when a voltage slightly higher than a threshold voltage is applied to a pixel electrode (portion encircled by a circle in the figure), the transmittance when viewed from the oblique direction becomes higher than the transmittance when viewed from the front. When the applied voltage is increased in some degree, the transmittance when viewed from the oblique direction becomes lower than the transmittance when viewed from the front. Thus, when viewed from the oblique direction, a brightness difference among a red pixel, a green pixel, and a blue pixel becomes small, and consequently, as described before, there occurs a phenomenon in which the screen looks whitish. This phenomenon is called “discolor”. The discolor occurs not only in the MVA liquid crystal display device but also in the TN liquid crystal display device.
U.S. Pat. No. 4,840,460 proposes that one pixel is divided into plural sub-pixels, and those sub-pixels are capacitance coupled to each other. In such a liquid crystal display device, since a potential is divided according to the capacitance ratio of the respective sub-pixels, voltages different from each other can be applied to the respective sub-pixels. Accordingly, seemingly, plural regions different in the threshold of the T-V characteristic exist in one pixel. When the plural regions different in the threshold of the T-V characteristic exist in one pixel as stated above, the phenomenon in which the transmittance obtained when viewed from the oblique direction becomes higher than the transmittance obtained when viewed from the front is suppressed, and consequently, the phenomenon (discolor) in which the screen looks whitish is also suppressed. The method in which one pixel is divided into plural capacitance-coupled sub-pixels to improve the display characteristic is called an HT (Half Tone gray scale) method. Incidentally, the liquid crystal display device disclosed in U.S. Pat. No. 4,840,460 is a TN liquid crystal display device.
Japanese Patent No. 3076938 (JP-A-5-66412) discloses a TN liquid crystal display device in which a pixel electrode is divided into plural sub-pixel electrodes, and a control electrode is disposed below each of the sub-pixel electrodes through an insulating film. In this liquid crystal display device, a display voltage is applied to the control electrode through a TFT. Since sizes of the respective sub-pixel electrodes are different from each other, voltages applied to the sub-pixel electrodes are also different from each other, and the effect by the HT method, that is, the effect to suppress the discolor can be obtained.
FIG. 3 shows a structure of one pixel of a conventional MVA liquid crystal display device using a capacitive coupling HT method. As shown in FIG. 3, a TFT substrate of the liquid crystal display device includes plural gate bus lines 12 formed on a glass substrate 10 (not shown in FIG. 3), and plural drain bus lines 14 formed to intersect with the gate bus lines 12 through an insulating film 30 (not shown in FIG. 3). A first TFT 20 formed as a switching element for each pixel is disposed in the vicinity of an intersection position of the gate bus line 12 and the drain bus line 14. A gate electrode 23 of the first TFT 20 is electrically connected to the gate bus line 12, and a drain electrode 21 is electrically connected to the drain bus line 14. A storage capacitor bus line 18 is formed to cross a pixel region defined by the gate bus line 12 and the drain bus line 14 and to extend in parallel to the gate bus line 12. A storage capacitor electrode (intermediate electrode) 19 is formed above the storage capacitor bus line 18 through the insulating film 30 for each pixel. The storage capacitor electrode 19 is electrically connected to a source electrode 22 of the first TFT 20 through a control capacitance electrode 25. A storage capacitor Cs is formed between the storage capacitor bus line 18 and the storage capacitor electrode 19.
The pixel region includes a sub-pixel A and a sub-pixel B. The sub-pixel A has, for example, a trapezoidal shape, and is disposed at the center of the pixel region and close to the left thereof. The sub-pixel B is disposed at the upper part, the lower part and the center right end part of the pixel region except the region of the sub-pixel A. Each of the sub-pixels A and B is substantially linearly symmetrical with respect to the storage capacitor bus line 18. A pixel electrode 16 is formed in the sub-pixel A, and a pixel electrode 17 separated from the pixel electrode 16 is formed in the sub-pixel B. The pixel electrode 16 is electrically connected to the storage capacitor electrode 19 and the source electrode 22 of the TFT 20 through a contact hole 24. On the other hand, the pixel electrode 17 is electrically in a floating state. The pixel electrode 17 has a region overlapping with the control capacitance electrode 25 through a protection film 31 (not shown in FIG. 3), and is indirectly connected to the source electrode 22 by capacitive coupling through a control capacitance Cc formed in the region.
A linear slit (blank part of an electrode) 44 extending obliquely to the pixel region end part is formed between the pixel electrodes 16 and 17. The slit 44 separates the pixel electrodes 16 and 17 from each other and functions also as an alignment regulating structure to regulate the alignment of a liquid crystal 6 (not shown in FIG. 3).
An opposite substrate disposed to be opposite to the TFT substrate through a liquid crystal layer includes a common electrode 41 (not shown in FIG. 3) formed on a glass substrate 11. A liquid crystal capacitance C1c1 is formed between the pixel electrode 16 of the sub-pixel A and the common electrode 41, and a liquid crystal capacitance C1c2 is formed between the pixel electrode 17 of the sub-pixel B and the common electrode 41. Linear protrusions 42 extending in parallel to the slit 44 and functioning as alignment regulating structures are formed on the common electrode 41. The linear protrusions 42 are disposed almost at the centers of the sub-pixels A and B in order to substantially equally divide the respective sub-pixels A and B into regions where alignment directions of the liquid crystal are different. The control capacitance electrode 25 to connect the source electrode 22 and the storage capacitor electrode 19 is disposed to overlap with the linear protrusions 42 when viewed vertically to the substrate surface. A light shielding film (BM) (not shown in FIG. 3) to shield the pixel region end part against light is formed on the opposite substrate.
It is assumed that the TFT 20 is brought into an on state, a voltage is applied to the pixel electrode 16, and a voltage Vpx1 is applied to the liquid crystal layer of the sub-pixel A. At this time, since the potential is divided in accordance with the capacitance ratio of the liquid crystal capacitance C1c2 and the control capacitance Cc, a voltage different from that of the pixel electrode 16 is applied to the pixel electrode 17 of the sub-pixel B. A voltage Vpx2 applied to the liquid crystal layer of the sub-pixel B becomes Vpx2=(Cc/(C1c2+Cc))×Vpx1. As stated above, in the liquid crystal display device having the pixel structure shown in FIG. 3, since the voltage Vpx1 applied to the liquid crystal layer of the sub-pixel A and the voltage Vpx2 applied to the liquid crystal layer of the sub-pixel B can be made different from each other in one pixel, the viewing angle characteristic can be improved.
Besides, in order to discharge an unnecessary storage charge which causes image sticking of the sub-pixel B, a second TFT 60 is provided. The gate bus line 12 serves also as a gate electrode of the TFT 60. An operational semiconductor layer (not shown) is formed on the gate electrode, and a channel protection film 28 is formed on the operational semiconductor layer. A source electrode 62 and a drain electrode 63 are formed on the channel protection film 28. The sub-pixel B is electrically connected to the source electrode 62 through a contact hole 64, and a portion extended from the control capacitance electrode 25 forms the drain electrode 63. The second TFT 60 is turned ON just before a specified voltage is written by the first TFT 20 into the respective pixel electrodes 16 and 17, and the pixel electrodes 16 and 17 are electrically connected to each other. Since the specified voltage is written into the pixel electrode 16 through the first TFT 20, an unnecessary charge is not cumulatively stored in the pixel electrode 17, and image sticking is prevented.
[Patent document 1] U.S. Pat. No. 4,840,460
[Patent document 2] Japanese Patent No. 3076938
The MVA type is designed such that when voltage is applied to liquid crystal molecules, the liquid crystal molecules are tilted by a slit or a bank, and the tilt is propagated so that the liquid crystal molecules of the whole pixel are tilted in a specified direction. However, when a propagation distance is long, there appears a liquid crystal molecule which falls in a direction different from the propagation direction before the alignment direction is propagated, and the alignment of the whole liquid crystal molecules is disturbed. Especially in a corner portion of each pixel, the propagation distance becomes long and the alignment is likely to be disturbed.
FIG. 3 shows a conventional example. A minute slit 46a is especially long at the corner portion of the pixel. Thus, the liquid crystal molecules on the minute slit 46a can fall in a direction opposite to the specified direction. A boundary between a region where the liquid crystal molecules fall in the specified direction and a region where the liquid crystal molecules fall in the direction opposite to the specified direction becomes a dark line. This dark line does not disappear unless all the liquid crystal molecules are re-aligned in the specified direction, and that can take several seconds or longer. When there is a pixel in which such a dark line is formed in a part of the liquid crystal display device, only the pixel looks slightly dark and causes a decrease in display quality, such as an afterimage.
Besides, since the potential of the gate bus line is lower than that of the pixel electrode by several volts to ten and several volts in almost all time, a large potential difference is generated between the gate bus line and the pixel electrode in the horizontal direction, and becomes one of factors of liquid crystal alignment disturbance. Although the second TFT 60 and the contact hole 64 of the sub-pixel A are provided in the vicinity of the gate bus line 12, as shown in FIG. 3, the minute slit can not be provided in the portion where the contact hole 64 is provided, and the portion is remote from the linear protrusion 42. Thus, liquid crystal molecules tilted in a direction different from the specified direction are increased. When the liquid crystal molecules are tilted in a direction different from the specified direction, a dark region is generated according to the tilt direction, or a boundary between regions where the liquid crystal molecules are tilted in different directions becomes a dark line. It takes several seconds before the liquid crystal molecules are re-aligned in the specified direction and the dark region or the dark line disappears, and that becomes a cause of a decrease in display quality, such as an afterimage.